Glass produced in the form of a sheet has a very broad range of uses including being used in architectural applications, automotive applications, liquid crystal display (LCD) applications etc.
Today several glass sheet forming processes are being used to manufacture glass sheets. Each of the glass sheet forming processes has limitations in terms of a working range and product properties which limit the domain of glass compositions that can be used to make glass sheets. One of the main product properties that limit the use of glass compositions is the liquidus temperature. The liquidus temperature corresponds to a practical viscosity below which the glass sheet forming process may not be maintained in continuous conditions.
The fusion draw process has the ability to produce a glass sheet with very good surface qualities, usable for instance in thin-film transistor (TFT) applications without needing prior surface polishing. The fusion drawn glass sheet which has pristine surfaces can be produced in the 0.1 to 3 mm thickness range. The limit of the fusion draw process, considering gravity effects, is that the glass sheet forming must take place with glass that has a viscosity which is higher than 40000 poises. This means that every glass composition which has a liquidus viscosity that is below 40000 poises cannot be practically formed into a glass sheet by utilizing the fusion draw process.
The rolling process produces a glass sheet that has surfaces which have traces of contact between glass and forming rolls. The rolled glass sheet can be produced in the 2 to 50 mm thickness range. The limit of the horizontally oriented version of the rolling process is that the glass sheet forming must take place with a glass that has a viscosity higher than 3000 poises. This means that every glass composition which has a liquidus viscosity that is below 3000 poises cannot be practically formed into a glass sheet by utilizing the horizontally oriented rolling process.
The float process produces a glass sheet which can be very large in size and can be manufactured at extremely high flow rates. The glass sheet which is produced by the float process has pristine surfaces and a thickness in the range of between 0.3 and 10 mm. FIGS. 1A-1B (PRIOR ART) illustrate a traditional float system 100 which has a structure 102 within which a molten glass 104 flows over a lip-stone 106 and free falls onto a liquid tin bath 108 and then flows over the liquid tin bath 108 to form a glass sheet 110. The traditional float system 100 also includes heaters 112 and an atmospheric opening 114. The traditional float system 100 is described in detail within an article by L. A. B. Pilkington “The Float Glass Process”, Proc. Roy. Soc. Lond., Vol. 314, pp 1-25 (16 Dec. 1969). The contents of this article are hereby incorporated herein by reference.
The float process generally works well when used to make glass sheets for architectural and automotive purposes. However, the float process also has a limitation where glass compositions which have a liquidus viscosity around 1000 poises cannot be practically formed into a glass sheet. This limitation occurs because the float process utilizes the lip-stone 106 to deliver the molten glass 104 to the liquid tin bath 108. In general, the lip-stone 106 can continuously spread glass with a viscosity around 3000 poises. However, due to a significant free surface flow on the lip-stone 106 before the molten glass 104 is delivered to the liquid tin bath 108 means that a significant volatization would occur in glass which has a viscosity in the 1000 poises range. This means that every glass composition which has a liquidus viscosity that is below 1000 poises will undergo volatization when formed into a glass sheet by utilizing the current float process. Thus, the float process needs to be adapted to improve the delivery of the molten glass 104 to the liquid tin bath 108 in order to be able to continuously form a glass sheet from a glass composition which has a liquidus viscosity less than 1000 poises. This need and other needs will be satisified by the present invention.